Conventional personnel dosimeters include instruments such as thermal luminescent devices for example. These devices use a small crystal of CaF2 or LiF for trapping the electrons and holes produced by the ionizing radiation. When heated, light is emitted from the crystal due to the emptying of the traps. This light is then related to the accumulated dose.
In another perspective, metal-oxide semiconductor dosimeters are MOS field-effect transistors with a specially processed gate insulator. The special gate insulator make the MOSFET radiation soft.
A MOSFET dosimeter measures a shift in the threshold voltage of a radiation field-effect transistor RADFET caused by radiation.
Canadian Pat. No. 1,204,885 which issued May 20, 1986 to Ian Thomson discloses a radiation dosimeter comprising a pair of silicon insulated gate field effect transistors (IGFET). This dosimeter operates by measuring the differential threshold between two IGFET sensors exposed to the same radiation, in which one is biased into its conducting region, and the other is biased either off or to a conducting level less than the first one. These dual IGFET dosimeters offer a sensitivity about 2 mV/cGy in the case where the gate bias is equal to 3 volts, or about 5 mV/cGy in the case where the gate voltage is greater than 10 volts. The temperature sensitivity of the dual IGFET sensor has been found to be smaller than 0.1 mV/° C. Over the temperature range of −20° C. to +50° C., a 70° C. difference, (such as in military applications) a ΔVT=7 mV or 1-3 cGy has been found.
The problem associated with this prior art device is that it is not sensitive enough for use by workers in the medical, nuclear and industrial fields, wherein a sensitivity of approximately 0.010 cGy (Rad) is required.
B. O'Connell, A. Kelleher, W. Lane, L. Adams in a paper entitled <<Stacked RADFETs for Increased Radiation Sensitivity>> published in IEEE Tran. Nucl. Sci. Vol. 43, N3, June 1996 have demonstrated a radiation sensitivity of 80 mV/cGy by stacking 15 individual RADFETs on the same chip.
V. Polischuk and G. Sarrabayrouse in a paper entitled <<MOS ionizing radiation dosimeters: from low to high dose measurement>> published in the revue of Radiation Physics and Chemistry, Vol. 61, No 3-6, 2001 presented a stack-connected RADFET configuration with RADFETs having a very thick gate oxide of 1.6 μm. In order to increase the sensitivity and the minimum measurable dose, up to 14 transistors have been stacked. The output voltage before irradiation was 18V. A sensitivity as high as 90 mV/cGy has been obtained.
Both teams experimenting with RADFETs claimed a possibility to measure milli-Rad doses. However, stacked RADFETs exhibit a number of problems which limit their use as personal dosimeters. The problem is that each RADFET has a certain temperature coefficient. The metal oxide semiconductor field-effect transistor device has a temperature threshold voltage dependence that needs to be accounted for in order that only radiation induced shift in threshold voltage is measured by the dosimeter. For stacked RADFETs the temperature sensitivity increases by a factor that is more than N times the sensitivity of a single one wherein N is the number of RADFETs in the stack.
G. Sarrabayrouse, D. Buchdahl, V. Polischuk, S. Siskos in a paper entitled <<Stacked MOS ionizing radiation dosimeters: potentials and limitations>> published in Radiation Physics and Chemistry, Vol. 71, 2003, pp. 737-739 have proposed to reduce temperature sensitivity of stacked RADFETs by measuring stacked RADFETs at the Minimum Temperature Coefficient (MTC) point. Indeed this paper presents only the computer simulations. The temperature sensitivity at MTC point and threshold voltage drifts were not measured.
Another problem associated with stacked RADFET resides in its high output voltage which in some cases is about 18 volts. Therefore it is difficult to amplify the small changes of threshold voltages, caused by radiation, by using operational amplifiers.